This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9921978.4 filed in the United Kingdom on Sep. 16, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to communication systems and in particular to radio frequency communication systems in which mobile stations communicate with base stations.
In a communication system including base stations and mobile stations which communicate with the base stations, the mobile stations include reference oscillators which are used to decode received transmissions from base stations and to transmit signals to the base stations. The mobile station reference oscillator provides a local frequency to the mobile station which, in theory, should be synchronised with and equal to the network frequency. However, due to inaccuracies in the crystal oscillators usually used in mobile stations, it is necessary for the mobile station to adjust its local frequency so that it is synchronised with the network frequency. Conventionally, complex algorithms are used which can operate only with high signal to noise ratios of the input network signal. The main cause of oscillator instability is temperature change and so temperature compensated high accuracy crystal oscillators can be provided. However, such a solution is considered to be expensive.
In addition to initial frequency synchronisation, a synchronised mobile station must update its local frequency to the network frequency because of the variance of local reference frequency with temperature.
Whilst the mobile station is in an active mode and is communicating with the base station, the mobile station has constant access to frequency and time reference information from the network, and can use this to make the required adjustments to local frequency. However, when a mobile station is in an idle mode, access to network information is limited. In an idle mode, the mobile station will occasionally be activated in order to receive paging signals from the network. In order to preserve battery power, the mobile station must only become active for a short amount of time. It is therefore necessary that the frequency and time reference estimation is carried out in a short amount of time. However, large estimation errors can occur when large variations of frequencies combine with low signal to noise ratios and short estimation periods.
In the proposed IMT 2000 system based on a wide band code division multiple access (W-CDMA) system-time boundary reference synchronization is required. Synchronizing to a time reference is dependent upon the accuracy of the local crystal frequency, and so a mobile station initially synchronises to the network by looking for a symbol, the so-called long code mask symbol (LCMS) which is transmitted in the broadcast channel (BCCH). The received signal is filtered by matched filters and peaks in the output of those filters provide the location of the LCMS. However, if the local oscillator crystal is inaccurate, for example xc2x110 ppm, the frequency deviation between the local crystal and the reference can be up to plus or minus 20 kHz. This is due to the carrier frequency being 2 GHz. Since the symbol rate in the BCCH is 16 kHz the symbol rotates 450xc2x0 (360xc2x0xc3x9710 ppmxc3x972 GHz/16 kHz) within the symbol duration. Such large rotation results in severe loss of the signal energy at the peaks of the matched filters output. Thus, the LCMS is divided to a number of sequences, for example 4, which are searched separately and the results of the match filters are non-coherently summed. Splitting the LCMS in this way reduces the symbol rotation for that portion of the LCMS. However, dividing the LCMS into four segments can also deteriorate the signal to noise ratio and can cause problems when detecting the full LCMS. The wideband CDMA specification (3rd Generation partnership Project (3 GPP) document TS25.201, V2.1.0) describes this system in more detail.
In an example of frequency synchronization, the global system for mobile communications (GSM) system transmits synchronisation information from base stations to mobile stations in terms of a frequency correction burst (FCB) and synchronisation burst (SB). The FCB consists of a pure sinusoid which needs to be detected, and which a mobile station uses to obtain rough time synchronisation to the network as well as to adjust the local frequency reference. The SB is transmitted 8 bursts after the FCB and so detection of the FCB enables the position of the SB to be determined. The FCB can also be used to provide a coarse adjustment of the reference frequency, while the SB is used for fine adjustment. This method is used in the GSM system, a description of which can be found in xe2x80x9cThe GSM system for mobile communicationsxe2x80x9d, by Mouly and Pautet.
There are a number of algorithms used for initial synchronisation of a mobile station to a GSM network.
Since the FCB is a pure sinusoid, a frequency selective filter is usually used to suppress the noise outside the frequency zone of interest in order to improve the signal to noise ratio. This improvement depends on the bandpass of the filter. The narrower the bandpass, the better the signal to noise ratio. The inaccuracy of the local crystal (for example 10 ppm) however can effect the frequency of the received FCB. This frequency can be interpreted with the deviation of +/xe2x88x929 KHz if the carrier frequency is 900 MHz. Hence the cut off frequency of the filter should be greater than 9 kHz. In a case of a carrier frequency of 1800 MHz or 1900 MHz, in the GSM 1800 and 1900 systems, the cutoff frequency of the filter would need to increase to 18 or 19 KHz. The limitation of the cut off frequency due to the inaccuracy of the crystal oscillator therefore reduces the improvement of signal to noise ratio and hence reduces the likelihood of fast synchronisation.
It is therefore desirable to provide an apparatus in which synchronisation of the local reference frequency to the network frequency can be achieved quickly and with minimum power consumption.
According to one aspect of the present invention, there is provided a method for synchronising a local reference frequency in a mobile station, the method including measuring a current ambient condition of the mobile station and using stored data indicating variance of the local reference frequency with known ambient conditions to determine the method by which the local reference frequency is synchronised.
According to another aspect of the present invention, there is provided an apparatus for communicating with a wireless network having a network reference frequency, the apparatus comprising a local oscillator for producing a signal having a local reference frequency, a storage device for storing data indicating the variance of the local reference frequency with changes in a known ambient condition of the apparatus, control means for adjusting the local frequency, and measurement means for measuring the ambient condition of the apparatus and supplying that measurement to the control means, wherein the control means is operable to retrieve data stored in the storage device which is indicated by the received measurement, and to adjust the local reference frequency in accordance with an adjustment method determined by the retrieved data.